Process for plating interior surface of tubing with cadmium sulfide



May 2, 1967 c. R. BARNES ET AL 3,317,342

PROCESS FOR PLATING INTERIOR SURFACE OF TUBING WITH CADMIUM SULFIDE Filed Dec.

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INVENTORS. CHARLES K-BAR/VES an (HA/HES 6565 [4 BY ATIQRNES United States Patent 3,317,342 PROCESS FOR PLATING INTERIOR SURFACE OF TUBING WITH CADMIUM SULFIDE Charles R. Barnes, Medway, and Charles R. Gecsner, New Carlisle, Ohio, assignors to the United States of America as represented by the Secretary of the All Force Filed Dec. 19, 1963, Ser. No. 331,979 5 Claims. (Cl. 117-5) .form continuous thin film of cadmium sulfide.

With the advent of space travel, it has been found necessary to produce photoelectric and photoresistive devices that are capable of being protected from the severe operational conditions encountered in the space environment. The utilization of a tubular type photoresistive device having an interior coating of cadmium sulfide meets the criteria of space travel.

Such tubes may be sealed with an inert gas and thus protected from the space environment. Heretofore, however, those skilled in the art have met with great difiiculty in attempting to deposit thin films of cadmium sulfide on the interior surfaces of fine bore tubes. Such films lack uniformity and continuity throughout the length of the tube resulting in the production of a device having photoresistive characteristics that vary from one portion of the tube to another. Such a device, of course, cannot be relied upon to produce a satisfactory result.

Accordingly, it is the primary object of this invention to provide a novel process for depositing a thin film of cadmium sulfide on the interior surfaces of a fine bore tube.

A further object of this invention is to provide a novel process for depositing a thin film of cadmium sulfide characterized by its uniformity and continuity throughout the interior surface of a fine bore tube.

Still a further object of this invention is to provide a novel process for producing a photoelectric or photoresistive device particularly adapted to be sealed and protected against the severe operational conditions encountered in the space environment.

Still further objects, advantages, andfeatures of this invention will become readily apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawing wherein the figure represents a schematic representation of an apparatus suitable for use with the process of this invention.

In accordance with this invention, it has been found that the aforementioned objects can be accomplished by directing a hydrogen carrier gas through an evaporating zone containing cadmium 2,4-pentanedione. The evaporating zone is maintained at a temperature of about 200 C. in order to effect evaporation of the cadmium 2,4-pentanedione and its transportation by the carrier gas into the interior portion of the tube to plated. The vaporous mixture is allowed to flow into the initial interior portion of the tube which is likewise heated to a temperature of about 200 C. in order to maintain the mixture in its vaporous state. The mixture is then directed into a second portion of the tube, hereinafter re ferred to as the plating zone. The plating zone is heated to a temperature of approximately 430 C. Hydrogen sulfide gas is directed into the plating zone and admixed with the hydrogen cadmium 2,4-pentanedione mixture, As a result of the high temperature, a reaction is effected between the vaporized cadmium 2,4-pentanedione and the hydrogen sulfide gas producing a cadmium sulfide reaction product which is subsequently deposited on the interior surface of the plating zone as a thin uniform continuous film. The remaining vapors are then directed through the last portion of the tube and subsequently expelled. The last portion is also maintained at 200 C. in order to prevent condensation of the vaporized cadmium 2,4-pentanedione in that portion of the tubing yet to be plated.

Heating of the evaporating zone and the tubing can be accomplished by any suitable heating means such as induction, resistance, dielectric or infra-red heating; or by passing the tube through heating zones or ovens maintained at the proper temperature. The use of Nichrome helixes carrying electric current, each controlled by a variable voltage supply, such as a Variac, has proved to be preferable.

This process has proved to be most effective and useful in producing a photoelectric or photoresistive coating on the interior surface of glass, quartz or Vycor tubing. During the plating process, the tubing may be maintained in a stationary position while the heating means is moved along the longitudinal axis of the tube. Alternatively, the heating means can be fixed in a stationary manner with the tube being moved along its longitudinal axis through an appropriate heating source. Movement of either the tubing or the heating means changes the position of the plating zone thereby eifecting deposition of the film throughout the tube.

Referring now to the drawing more in detail, there is schematically disclosed a suitable apparatus for accomplishing the process of this invention. Hydrogen gas from a compressed gas cylinder 10 is directed through a control valve 12 and a flowmeter 14 at a flow-rate of 400 milliliters per minute. The hydrogen next flows through a deoxidizing unit 16 where it comes in contact with a supported palladium sponge which catalytically converts any contaminating oxygen into water vapor. The vapor is subsequently removed by a conventional molecular sieve drier 18. The dry hydrogen gas then enters an evaporator 20 which contains cadmium 2,4-pentanedione designated at 22. The temperature of the evaporator 20 is maintained at approximately 200 C. by conventionally controlled electrical heating coils 24 which cause evaporation of the cadmium containing compound 22 and its transportation by the hydrogen gas through a rubber connector 26 to the interior portion of a Vycor tube 28. Hydrogen sulfide gas from a compressed gas cylinder 30 and controlled by valve 32 flows through a wire tube 34 at the rate of 50 milliters per minute as measured by a flow-meter 36. The hydrogen sulfide is directed by tube 34 through rubber connector 26 into the plating zone 38 on the inside of the Vycor tube 28 at a point near clamp 40. The outside diameter of the wire tube 34 is less than one millimeter.

The plating zone 38 is heated to approximately 430 C. by means of a Nichrome helix 42 which carries electric current controlled by a variable voltage supply 44, such as a V-ariac. The length of the plating zone 38 is approximately one foot and is determined by the distance which the clamps 40 and 46 are placed apart. Within the plating zone 38, the hydrogen sulfide gas and the vaporized cadmium 2,4-pentanedione react to form cadmium sulfide which is plated as a clear adherent amber colored thin film on the interior surface of the Vycor tube 28. During the plating process the Vycor tube 28, between clamps 40 and 48 and clamps 46 and 50, is maintained at a temperature of 200 C. by Nichrome one foot distance.

coils 56 in order to prevent condensation of the cadmium 2,4-pentanedione vapors in those portions of the tube yet to be plated. After plating the remaining vapors are expelled through exit 54.

In the actual plating process, the plating zone 38 is set up at the right end of the tubing to be plated and is moved at the rate of 4 inches per hour toward the left or fore end of the tube by moving the tube through clamps 40 and 46 to the left while keeping them one foot apart. At the same time the wire tubing 34 which supplies the hydrogen sulfide gas to the plating zone 38 is also moved at the same rate so that its tip or discharge end is always at a point under clamp 40. The unplated portion of tube 28 would be maintained at all times at a temperature of about 200 C. as controlled by the adjustment of a variable voltage supply 52, such as Variac.

For movement of the tube, as described above, the coils 56 would be made long enough to assure heating of the tube outside-of the plating area.

Through the utilization of this process, a section of a Vycor glasstube of two foot length having an inside diameter of of an inch was satisfactorily plated with a continuous and uniformly thin film of cadmium sulfide of about 0.3 mil thickness in six hours. However, the process of this invention is not limited to diameters of of an inch, but can be utilized in plating the interior surfaces of smaller or larger tubing as desired.

The Vycor tubing plated as above with an 0.3 mil thick coating of cadmium sulfide was tested for its photoelectric properties in accordance with the following procedure. The coated tube was removed from the apparatus and cut into sections of of an inch in length. Platinum lead wires were attached to each end of the film of cadmium sulfide in order to form a photoelectric cell. These cells were alternately placed in darkness and in a parallel beam of light from a 1600 candle power lamp held at The resistance of the cells was then measured. The resistance in the dark was 500,000 ohms while in light it measured 500 ohms. In other words, the ratio of resistance in going from darkness to the illumination of a 1600 foot candle intensity was 1000 to one.

The uniformity and continuity of the cadmium sulfide coating produced by the process of this invention provides a photoresistive device which is characterized by its reliability and uniformity of result. Consequently, the photoelectric cells produced by this invention have been found to be especially useful in guidance systems of satillites and missiles, as well as other electronic systems, where reliability is of paramount importance.

It will be understood by those skilled in the art to which the present invention pertains, that while the process disclosed herein illustrates a preferred embodiment of the invention, various alterations and modifications can be made without departing from the spirit and scope thereof, and that all such modifications as fall within the scope of the appended claims are intended to be included herein.

What we claim is:

\1. A process for plating the interior surface of a fine bore tube comprising the steps of directing a vaporized mixture of hydrogen, cadmium 2,4-pentanedione, and hydrogen sulfide through the length of said tube, heating said tube containing said mixture to a first temperature sufficient to maintain said mixture in the vaporous state, said mixture being in contact with substantially all of the interior surface of said tube, heating a zone of said tube to a second temperature higher than said first temperature, said second temperature being sufficient to form a reaction between the cadmium 2,4-pentanedione and the hydrogen sulfide and effect deposition of a thin, uniform, continuous film of cadmium sulfide on the interior surface of said zone of said tube.

2. A process for plating the interior surface of a fine bore tube comprising the steps of supporting said tube along its longitudinal axis for longitudinal movement, directing a vaporized mixture of hydrogen, cadmium, 2,4- pentanedione and hydrogen sulfide through the length of said tube from one end thereof to the other end thereof, heating said tube containing said mixture throughout its length to a first temperature sufiicient to maintain said mixture in the vaporous state, said mixture being in contact with substantially all of the interior surface of said tube, heating a zone of said tube to a second temperature, said second temperature being higher than said first temperature and sufficient to form a reaction between the cadmium 2,4-pentanedione and the hydrogen sulfide and effect deposition of a thin, uniform, continuous film of cadmium sulfide on the interior surface of said zone of said tube..

3. A process in accordance with claim 2 including the step of moving said tube along its longitudinal axis in a direction from said other end thereof to said one end thereof in order to effect a change of position of said zone of said tube whereby a thin uniform, continuous film of cadmium sulfide is deposited upon the unplated portion of the interior surface of said tube.

4. A process for plating the interior surface of a fine bore tube comprising the steps of supporting said tube in a stationary manner within a housing defining a first heating zone and a second heating zone, said housing supported for longitudinal movement along the longitudinal axis of said tube, directing a vaporized mixture of hydrogen, cadmium 2,4-pentanedione and hydrogen sulfide through the length of said tube from one end thereof to the other end thereof, said mixture being in contact with substantially all of the interior surface of said tube, maintaining said first zone at a temperature sufficient to heat said tube to a point wherein said mixture is maintained in the vaporous state, maintaining said second zone at a temperature higher than the temperature of said first zone in order to heat that portion of the tube in said second zone to a point sufficient to form a reaction between the cadmium 2,4-pentanedione and the hydrogen sulfide and effect the deposition of a thin, continuous, uniform film of cadmium sulfide on the interior surface of said tube at the situs of said zone.

5. A process in accordance with claim 4 including the step of moving said second heating zone along the longitudinal axis of said tube from said one end thereof to said other end thereof in order to effect a change in the position of that portion of the tube in the said second zone whereby a thin, uniform, continuous film of cadmium sulfide is deposited on the unplated portion of the interior surface of said tube.

References Cited by the Examiner UNITED STATES PATENTS 2,659,678 11/1953 Cusano et al 117-106 2,675,331 4.41954 Cusano et al. 117-106 2,755,172 7/ 1956 McGauley et al. 23134 2,980,500 4/ 1961 Miller 23-134 X 3,127,282 3/ 1964 Hershinger 117106 X ALFRED L. LEAVITT, Primary Examiner.

A. G. GOLIAN, Assistant Examiner. 

1. A PROCESS FOR PLATING THE INTERIOR SURFACE OF A FINE BORE TUBE COMPRISING THE STEPS OF DIRECTING A VAPORIZED MIXTURE OF HYDROGEN, CADMIUM 2,4-PENTANEDIONE, AND HYDROGEN SULFIDE THROUGH THE LENGTH OF SAID TUBE, HEATING SAID TUBE CONTAINING SAID MIXTURE TO A FIRST TEMPERATURE SUFFICIENT TO MAINTAIN SAID MIXTURE IN THE VAPOROUS STATE, SAID MIXTURE BEING IN CONTACT WITH SUBSTANTIALLY ALL OF THE INTERIOR SURFACE OF SAID TUBE, HEATING A ZONE OF SAID TUBE TO A SECOND TEMPERATURE HIGHER THAN SAID FIRST TEMPERATURE, SAID SECOND TEMPERATURE BEING SUFFICIENT TO FORM A REACTING BETWEEN THE CADMIUM 2,4-PENTANEDIONE AND THE HYDROGEN SULFIDE AND EFFECT DEPOSITION OF A THIN, UNIFORM, CONTINUOUS FILM OF CADMIUM SULFIDE N THE INTERIOR SURFACE OF SAID ZONE OF SAID TUBE. 